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About this Article
Written by: Mark Villanueva
Written on: December 9th, 2002
Tags: physics
Thumbnail by: AllenMcC./Wikimedia Commons
About the Author
At the time of publication, Mark Villanueva was a student at the University of Southern California in his third year towards his BS in Computer Science. Back to the Future is one of his favorite movies.
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Volume V Issue I > The Science of Time Travel
Time travel involves either moving backward to the past or forward to the future. Just as our current actions affect the future, our actions of the past can affect the present. The Grandfather Paradox, which must be addressed when discussing time travel, describes the potential problems that can arise from traveling to the past. Albert Einstein resolved the issue with the concept of world lines. Future time travel requires traveling at the speed of light while past time travel requires the use of wormholes. Both types of time travel are very difficult to accomplish because of the technology and energy that would be required to move at the speed of light. While theoretically possible, time travel is better left to a more advanced civilization.

Introduction

In a modern world inhabited with robotic dogs, the Internet, and cloned mice, science fiction is quickly becoming science fact. One of the most intriguing science fiction technologies is time travel. Who wouldn't want to go to the past to relive the best day of their life or travel to the future to witness the first colony on Mars? Time travel involves more than merely jumping into a time machine and entering the desired date of arrival. Even though the reality of time travel is theoretically possible, it cannot be fully implemented in the near future due to massive energy requirements. In addition, the existence of time travel creates paradoxical issues that need to be resolved. However, time travel is not totally out of the question.

Definition of Time Travel

To better understand the possibility of time travel, it is important to first define it. In our world, we can move freely in three spatial dimensions [1]: forward and backward; left and right; up and down. We can accomplish movement in any of these directions without a second thought. For example, I can travel to my professor's office by walking down one street, making a left onto an intersecting avenue, and then taking an elevator up to the second floor (Fig. 1). Every object's position around the world can be described by these three dimensions. There also exists an additional dimension, that of time, which physicist Albert Einstein established as the fourth dimension [2].
Illumin
Figure 1: Time is considered the fourth dimension in which we can only move forward
Traversing in the fourth dimension is vastly different than moving in the previously mentioned three. We simply cannot will ourselves to move forward five minutes or back ten days. In other words, time only moves forward, and we are stuck moving in that direction like corks bobbing helplessly in a river [1]. Thus, the end goal of time travel is to enable us to control where we go in this fourth dimension. Much like moving back and forth, time travel involves moving to either the past or the future. However, our actions in the fourth dimension are cause for concern. What we do now in the present affects our future. In the same manner, our actions in the past should have affected our present lives. Changing the outcomes of past events leads to what physicists and philosophers refer to as the "Grandfather Paradox," an issue that needs to be addressed in any serious discussion of time travel.

The Grandfather Paradox

Suppose time travel to the past does one day become possible and some time travelers make a journey to 1960s Texas to prevent the assassination of President Kennedy. Another time travel group brings back blueprints of the latest SUV to a young Henry Ford. Logic dictates that these meddling time travelers just altered the future. What if by changing the future world, they prevented themselves from being born? This would mean that they should cease to exist, but if they were not born, then how did they change the past in the first place? These seemingly mind-numbing problems are variations on the Grandfather Paradox.
The Grandfather Paradox involves the question: If a person traveled to the past to murder his grandfather, thus preventing his father and subsequently himself from being born, what happens to that person? Does he simply fade away into non-existence? If he is not born in the first place, how did he manage to travel back in time to kill his grandfather? A solution to this paradox is necessary before even considering the possibility of time travel. There are a number of suggestions of ways to resolve the paradox. Philosophers present a simple solution. They reason that when the grandson attempts to murder his grandfather, he fails on all accounts because it is logically impossible to change the past [3]. This is because the grandson still exists in the present and when he goes back in time. Any number of things could go wrong: he misses on every shot or the gun doesn't work. By virtue of his own existence, no matter how hard he tries, he cannot kill his grandfather. This can also be applied to other cases where time travelers attempt to change the past. They simply cannot succeed due to the nature of the timeline from which they came.
The philosophical drawback to this resolution is that it has serious implications on the nature of free will [4]. The grandson freely decides to kill his grandfather, yet it is as if some force has predetermined that his grandfather will not die. The absence of free will on the grandson's part makes this solution to the paradox hard to accept. Another explanation of the paradox involves the concept of parallel universes.
Theoretical physicist David Deutsch proposes the concept of "many worlds" in resolving the Grandfather Paradox [1]. He theorizes that multiple universes are created by each event in the world. For instance, suppose I waited until the last minute to write this article instead of starting on time. This decision just triggered another universe. In one world, I started the article on time and received a passing grade. In one of the many others, the paper was late and I was not pleased with my grade. Applied to the Grandfather Paradox, if the grandson was successful in killing his grandfather, it would create an alternate universe in which his grandfather was dead. This would in no way affect the grandson, and he would still remain in existence, passing from one universe to another. So the original universe with the living grandfather still exists yet the grandson is absent from it. The grandson instead lives in the newly created universe without the grandfather.
This solution to the paradox has its flaws, however. Every single situation for every single person must be accounted for, resulting in an infinite number of different worlds. In fact, the smallest change in the most minuscule of atoms could spur another world into existence. This many-worlds solution is difficult to accept because it mandates "the existence of a mind-boggling number of parallel universes that accommodate every possible combination of behaviour for all the particles in the universe" [5].
Einstein"s theory of relativity gives the best answer to the Grandfather Paradox by providing the idea of a "world line." A world line is best visualized on a time versus distance graph. Your position at any time is plotted in terms of time and distance: the time you are at that position and its distance from an origin. If I remain in my apartment (the origin) for two hours, for example, the world line is a vertical line parallel to the y-axis. No distance was traveled but time elapsed. If I walk to school, my world line becomes slanted because not only is time increasing, I also travel a distance to school. In this fashion, one can imagine a long world line being traced over the span of my lifetime, from my birth until my death. In fact, after death, my world line will not end. The world lines of the molecules in my body will continue because they too travel a distance over a time [2].
This world line view can now address the Grandfather Paradox. If the grandson manages to kill his grandfather, the commonly held belief is that the grandson immediately fades away into non-existence. Since his grandfather dies, his father is never born, which precludes his own existence. If that were the case, his world line would disappear, which by Einstein's theory, is impossible. As mentioned before, world lines do not end, and cannot be destroyed. Each world line can be broken down into smaller world lines, world lines of molecules, for example, but they can never be cut. So, in actuality, you could kill your grandfather in the past and it would have no bearing on your current state.

"Building" a Time Machine

It is important to recognize that the theoretical specifics on how to accomplish time travel are highly speculative. Nevertheless, there are two directions of time travel: going to the past and going to the future. The latter is the easiest to perform, relatively speaking of course.
Einstein"s Theory of Special Relativity states that "time slows down for objects moving close to the speed of light, at least from the viewpoint of a stationary observer" [1]. For instance, an astronaut can travel 500 light years into outer space at 99% the speed of light and return (at the same speed) to an earth that will be 1000 years older while the astronaut will only have aged 10 years [4]. Experiments have been performed wherein atomic clocks were sent into orbit around the earth and their ticks were compared to clocks on earth. The clock on earth and the clock in space ticked at different rates [2]. Thus, time passes differently when you move in space; traveling into the future is possible, even at the speeds that our astronauts travel. Of course, they would probably only be gaining a few nanoseconds, but when they return, the time on earth is ahead of their system time. From their perspective, they have returned to the future [6]. This time dilation effect is more pronounced when velocity approaches the speed of light. Unfortunately, at the speed of light, time stops, the length of the moving object contracts to zero, and resistance to acceleration becomes infinite, requiring infinite energy [6]. Obviously, it is physically impossible to travel at the speed of light, let alone beyond the speed of light, a logical requirement for traveling to the past.
Theories of time travel into the past utilize the notions of black holes and wormholes. A black hole is a celestial body whose surface gravity is so strong that nothing can escape from within it. In theory, a black hole comes about when a star undergoes gravitational collapse at the end of its star cycle. The concept of black holes goes back to the French mathematician Pierre Simon de Laplace. In a 1798 treatise, Laplace agreed with Newton's assertion that light is composed of particles. He reasoned that if enough mass were added to a star (such as our Sun), the gravitational force of the star eventually would become so great that its escape velocity, the minimum speed required for an object to escape permanently from the star's gravitational field, would equal the velocity of light. At that stage, light particles would not be able to leave the surface of the star and it would become an invisible star. More than a century later, Einstein, developing his special theory of relativity, asserted that nothing can move faster than light. Therefore, Laplace's black stars are also black holes, because if light cannot escape, all other matter must be trapped as well. The surface of a black hole thus acts like a one-way membrane: material may fall into a black hole, but no information or energy can come out of a black hole [7] (Fig. 2).
AllenMcC./Wikimedia Commons
Figure 2: A theoretical depiction of a black hole, from which nothing--even light--can escape.
Unfortunately​, no black hole has yet been positively identified. Black holes, if they exist, could come in an extreme range of sizes. The English physicist Stephen Hawking has speculated that tiny black holes with masses no larger than that of a large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under the extreme conditions that cosmology theories indicate existed in the very first moments of the universe. On the other hand, gigantic black holes may lie at the center of galaxies [7]. Einstein envisioned a situation in which the ends of two different black holes could be connected. This is known as a wormhole.
Unfortunately, no black hole has yet been positively identified. Black holes, if they exist, could come in an extreme range of sizes. The English physicist Stephen Hawking has speculated that tiny black holes with masses no larger than that of a large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under the extreme conditions that cosmology theories indicate existed in the very first moments of the universe. On the other hand, gigantic black holes may lie at the center of galaxies [7]. Einstein envisioned a situation in which the ends of two different black holes could be connected. This is known as a wormhole.
AllenMcC./Wikimedia Commons
Figure 3: A theoretical depiction of a wormhole. Scientists postulate that with these, a quick path through the universe can be taken.
The wormhole is one basis for time travel into the past. Physicists liken wormholes to quick paths through the universe, much like the hole a worm burrows through an apple [8]. Instead of apples, these wormholes are theoretical tunnel shortcuts through space (Fig. 3).The trick in this case would be flying a spaceship into the one mouth of the wormhole and coming out the other side in a different time and place [9]. This involves moving one end of the wormhole close to the speed light and keeping the other end stationary near earth in outer space. Like the example with the spaceship traveling near the speed of light in space, the moving end of the wormhole in space will "age" slower than the stationary one; the "younger" end is a quick shortcut that connects to an earlier time on the fixed end [1], so the moving end of the wormhole will bring the traveler back to the past. The time travel process will probably involve a team of advanced scientists that can create wormholes and move them while the time traveler goes through the wormhole in outer space via a spaceship.
Even though a wormhole could be used to travel to the past, the concept is still based on theory. In 1917, German astrophysicist Karl Schwarzschild used Einstein's theory to calculate that if a star of any given mass were compressed to a size smaller than a critical radius, now called the Schwarzschild radius in his honor, the density would become so high and the gravitational force so great that the star would become a black hole. The spherical surface around the star at the Schwarzschild radius is called the "event horizon" and marks the outer surface of the black hole at which the escape velocity just equals the velocity of light [7]. Suffice it to say, scientists cannot generate wormholes, let alone move them at the speed of light and position them at will. Forming a wormhole requires a mass equivalent to two hundred times the mass of our sun. Moreover, there is the problem of a traveler surviving the wormhole's gravitational field, which would easily tear the traveler apart [8]. Another problem with a wormhole is that the channel created between the two black holes is smaller than a single atom and only remains open for a fraction of a second [10].
One way to keep the wormhole open long enough for a spaceship to travel through is by using antimatter. Antimatter is a form of matter in which the electrical charge of each constituent particle is the opposite of that in the usual matter of our universe. That is, an atom of antimatter has a nucleus of antiprotons and antineutrons surrounded by positrons. Regular matter has the tendency to draw other bodies of mass to it. For example, massive objects, like the sun, pull planets into its orbit. Antimatter has the opposite effect. It, unlike regular matter, pushes the space around it apart [11]. Thus, antimatter could be used to hold open the "throat" of the wormhole long enough for a spaceship to travel through [10].

The Reality of Time Travel

Time travel is vastly different than what is portrayed in popular fiction. To dash the hopes of anxious time travelers, don't expect to be witnessing Napoleon's last stand or hoverboarding anytime soon. First, only a highly advanced civilization can meet the energy requirements and possess the technology needed for time travel. Such a civilization will need to harness the power of stars and be able to exploit entire galaxies as power sources [2]. Second, time travelers cannot alter their current state by traveling to the past. They can observe and interact but it will make no difference on the future. Finally, a time machine is not a self-contained unit such as a chair surrounded by rotating gears or a souped-up Delorean: real time travel involves spaceships, black holes, and wormholes. One cannot merely push buttons and travel through time. While time travel may not be realized in our lifetimes, it may be possible far off in the future. In the meantime, fanciful tales of time travel still make for a good read.

References

    • [1] J. R. Gott. "Will We Travel Back (Or Forward) In Time?" Time, Apr. 10, 2000: pp. 68.
    • [2] M. Kaku. Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension. New York: Anchor Books, 1994.
    • [3] P. J. Riggs. "The Principal Paradox of Time Travel." Ratio X, Apr. 1, 1997: pp. 49-64.
    • [4] S. Mowbray. "Let's do the time warp again." Popular Science]/i], Mar. 2002: pp. 46-51.
    • [5] "How to murder your grandfather and still get born." The Economist, Jan. 20, 1996: pp. 81.
    • [6] J. M. Zavisa. "How Special Relativity Works" Internet: http://www.howstuffw​orks.com, May 2, 2003. [Oct. 18, 2002].
    • [7] L. Smarr. "Black Hole". The New Grolier Multimedia Encyclopedia. CD-ROM. 1993 Grolier Electronic Publishing, Inc.
    • [8] A. Ramirez. "Clockwork: time travel isn't what it used to be." The New York Times Jul. 28 2002, natl. ed.: pp. WK3.
    • [9] S. W. Hawking. "Protecting the Past: Is Time Travel Possible?" Astronomy, Apr. 2002: pp. 46.
    • [10] M. Moyer. "The Physics of Time Travel." Popular Science, Mar. 2002: pp. 52-53.
    • [11] C. J. Wheeler. "Of wormholes, time machines and paradoxes." Astronomy, Feb 1996: pp. 52-58.